Allogeneic hematopoietic cell transplantation (HCT) is an important treatment modality for patients with both malignant and nonmalignant hematologic disorders. However, the application of this treatment has been limited to relatively young patients by complications related to the toxicity of the conditioning regimens used. To decrease toxicities, nonmyeloablative regimens have been developed. While these have been quite successful in major histocompatibility antigen complex (MHC)-identical transplants, where 200 cGy total body irradiation (TBI)is adequate for engraftment, in the more complex MHC-haploidentical setting much higher and toxic TBI doses are required to ensure engraftment. We propose to investigate a systemically targeted form of radiation to replace TBI in both MHC-identical and MHC-haploidentical HCT which would provide a treatment option for patients without MHC-matched donors. Specifically, the research efforts will determine if the a-emitting radionuclide 211 Astatine (211At), when conjugated to a panhematopoietic anti- CD45 monoclonal antibody (MAb), can replace TBI to condition recipients for allogeneic HCT. We will utilize our well-established preclinical model of randombred dogs which has been predictive of allogeneic HCT in humans. In specific aim 1, we will evaluate and optimize a new method for labeling MAbs with 211At. The most effective method for labeling MAbs with 211At used currently involves a two-step process where a stannylbenzoate ester is astatinated, then conjugated with the MAb.The method to be investigated will involve conjugation of a molecule containing an astatine-reactive borate(2-) moiety to the MAb, followed by astatination. Potential advantages of the new labeling method include less handling of radioactive materials, as well as higher and more consistent labeling yields. In specific aim 2, the efficacy and toxicity of 211At- labeled anti-CD45 MAbs will be compared with MAbs labeled with another a-emitting radionuclide, 213Bismuth (213Bi) on CD45-expressing cells and in a mouse model. The comparative studies will allow use of information obtained in prior studies with 213Bi-labeled MAbs to help determine initial conditions and quantities for the 211At studies. In specific aim 3, evaluations of 211At-labeled MAbs will be conducted in dogs to find effective doses for both dog leukocyte antigen (DLA)-identical and DLA-haploidentical HCT.Initially, dose-finding studies will be conducted to determine the minimal dose that is effective for myelosuppression. Following that, quantity of 211At-labeled anti-CD45 MAb required to obtain stable engraftment in HCT involving DLA-identical littermates will be determined. The final, and most important, studies will involve determining the quantity of 211At-labeled MAb required to obtain stable engraftment in HCT involvingDLA- haploidentical littermates.